We have always been at war

In the 2014 State of the Union address, President Obama reiterated his promise to bring our troops home from "America's longest war".

"Longest war huh? That's a hell of a thing" my inner monologue replied.

Curiosity once again got the best of me, and I wondered: how long does war normally last? Let's find out!

I scraped wikipedia for this project, which I did clumsily in my post-Seahawks victory euphoria. Just looking at America's history of war was boring/limited, since we haven't fought in that many. I found a great category to use: Lists of Wars by Date, and grabbed all the wars I could from 1000BC to 2002AD. (For reference, here's a handy wikipedia page on length of U.S. participation in various wars.)

After a little data wrangling, my sample included 1,185 individual wars, which also includes major conflicts not officially categorized as a "war" (e.g. the US involvement in Vietnam). While the data is probably full of small errors, and overly Euro-centric, this seemed like an ideal sample to answer my questions!

Here's the histogram of the duration of wars throughout the last 3000 years of human history:

For all wars that began and ended within a single calendar year I flatly assumed a duration of 6 months. The average (mean) duration of war is 10.4 years, but that distribution is severely lopsided! The median war duration is just 2 years.

Conclusion 1: Most wars are fairly short

Now to the war that prompted this question: Afghanistan. America has been fighting this war (by most definitions) for over 12.3 years. I can only discuss the cost in time, not in life or treasure, nor opportunities lost. In this next figure I've sorted every war by it's duration, and then created a reverse cumulative distribution. This figure is a little funky, since there are many wars for a given duration (hence the vertical dark blue lines). In retrospect I'd probably just trace the top boundary of the light blue region.

The way to read this is as follows:

100% of wars are 6months or longer,

75% of wars are 1 year or longer,

56% of wars are 2 years or longer, and so on....

Conclusion 2: The current U.S. (& allied) war in Afghanistan is longer than 84% of all wars... ever!

"But wait, X war was way longer" you protest!

Of course! Some wars lasted centuries. Thankfully most don't....

Finally, with a sample of over 1,100 wars spread over 3000 years, and an average of 2 years per war, I wondered if there was ever a time without a war. These data naturally becomes less complete the further back in time you go. They also appear to be biased towards histories of "western" people (e.g. probably many wars are unlisted in this dataset from ancient Africa, the Americas and some timespans in Asia). As such, consider this a lower limit. With all that in mind...

Conclusion 3: the most recent time in history with no wars occurring was 597 AD!

This is an unlikely result for many reasons (even beyond the shortcomings of the data). The year 597 AD started on a Tuesday, and the "lack of war" was more of an intermission. The Romans had an "interlude" to a then decade-long war in the Balkans. The following year (or maybe that same year) it reignites, and the war continues for another ~10 years.

You have to go back more than 1400 years ago to find a year without a recorded war in our data. It seems reasonable to believe that not once in the past 3000 years has humanity been "war free". And if a war-free year did occur, it could only have been by dumb luck.

But wait, there's hope...

Considering the human population over time (shown below for most of human civilization), our rate of wars per capita is overall decreasing with time!